Why is the sky blue?

The sky is blue because of how sunlight interacts with our atmosphere. Sunlight looks white but is really a mix of all colours, each travelling as a wave of a different length. As sunlight passes through the air, it strikes tiny gas molecules that scatter it in every direction. Shorter, bluer wavelengths scatter far more strongly than longer, redder ones, an effect known as Rayleigh scattering. So blue light gets bounced around the sky and reaches your eyes from all directions, making the whole sky look blue. At sunrise and sunset, the light travels through much more atmosphere, which scatters the blue away and leaves the warm reds and oranges we see.